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Response of Fattening Ewe Lambs to Oral Melengestrol Acetate^{1, 2,}

Texas Tech University, Lubbock

Abstract

Ewe lambs fed melengestrol acetate at daily levels of 0.04, 0.08, and 0.16 mg made less gains than control lambs, however, the 0.24 mg MGA-treated groups gained more (0.14 vs. 0.13 kg daily) than controls. Treated lambs required more feed per unit of gain in trial I, but in trial II, the reverse occurred and control lambs utilized more feed per unit gain. Treated lambs consumed less feed than controls in both trials. Carcass grade, dressing percent and fleece production were unaffected by MGA.

The effect of MGA on genitalia was more pronounced, although uterine weights were not affected. In the treated lambs, cervical weights were significantly (P<.01) larger, ovarian weights were smaller, ovaries had fewer follicles but not a lesser number of large follicles, and the number of corpora lutea were present in infinitely small numbers in one trial and absent in the other. The data on corpora lutea suggest that lambs at 30 kg body weight are prepuberal. From these data it appears that MGA may have exerted an inhibitory influence on both follicle development and ripening, and suppressed the development of corpora lutea, or that lambs, unlike heifers, may have failed to respond because of a species difference.

¹ This research was supported in part by a grant-in-aid from The Upjohn Company, Kalamazoo, Michigan.

² Contribution No. 33, International Center for Arid and Semi-Arid Land Studies.

³ Department of Animal Science.

⁴ Present address: College of Veterinary Medicine, Texas A&M University.